This invention is a process for removing nitrogen from natural gas, and is especially useful when the nitrogen content of the natural gas stream varies considerably over time, and when the natural gas stream also contains a sizeable concentration of heavy hydrocarbons.
Recovery of high quality natural gas is becoming increasingly important as the price of energy continues to rise. Furthermore, natural gas usage tends to minimize the quantity of pollutants produced for a given amount of energy generated when compared to certain other commonly used means of energy generation.
One problem often encountered in natural gas recovery is nitrogen contamination. Natural gas which has significant amounts of nitrogen may not meet minimum heating value specifications, reduces pipeline capacities and requires additional compression horsepower and fuel consumption. Further, nitrogen in natural gas may lead to the formation of undesired nitrogen oxides when the natural gas is burned. Nitrogen removal from natural gas has therefore attained increased importance.
In many cases, successful recovery of the natural gas requires the use of an enhanced recovery technique. One such often used technique involves the injection into the well of a fluid which will not support combustion; an often used fluid for this technique is nitrogen due to its relatively low cost compared to argon, helium and the like. However, the use of this technique increases the level of nitrogen contaminant in the natural gas above the naturally-occurring nitrogen concentration.
Nitrogen injection for enhanced recovery introduces a further problem because the nitrogen concentration in the natural gas does not remain constant over the life of the recovery operation. During the first few years that enhanced recovery with nitrogen injection is employed, the nitrogen concentration in the natural gas may remain at about the naturally-occurring level, increasing thereafter, for example, by about 5 percent after 4 years, by about 15 percent after 8 years, by about 25 percent after 10 years and by about 50 percent after 16 years.
In response to the problem of nitrogen contamination of natural gas, several methods of separating the nitrogen from the natural gas have been developed. A commonly used method employs a dual pressure double distillation column; this type of arrangement is often used in the fractionation of air into oxygen and nitrogen. However, this method is generally limited to applications where the nitrogen concentration of the natural gas is greater than about 20 to 25 percent. Where the nitrogen concentration is lower than 20 to 25 percent, the quantity of reflux liquid that can be generated in the high pressure column decreases to the extent that proper fractionation cannot be conducted in the low pressure column.
Generally, prior methods of cleaning natural gas of nitrogen at such low nitrogen concentration have been designed for natural gas having low concentrations of heavy hydrocarbons and/or a relatively unchanging nitrogen concentration in the natural gas.
The problem of a changing nitrogen concentration in the natural gas further complicates the economics of recovery. As shown, for example, in "Design Considerations For Nitrogen Rejection Plants," R. A. Harris, Apr. 17, 1980, The Randall Corp., Houston, Tex., the specific nitrogen removal process employed will be dictated by the nitrogen concentration. A nitrogen concentration of from 15 to 25 percent will require one type of process, a nitrogen concentration of from 25 to 40 percent will require another, a nitrogen concentration of 40 to 50 percent still another process, and a concentration greater than about 50 percent yet another process. The alternative, i.e., the use of only one process as the nitrogen concentration in the natural gas varies, will result in severe operating inefficiencies.